Formation mechanism of HA-based coatings by micro-arc oxidation

Abstract

Hydroxyapatite (HA)-based coatings were prepared by micro-arc oxidation (MAO) in an aqueous electrolyte containing calcium acetate and β-glycerophosphate disodium at high applied voltage. The structure and evolution of phase components in the coatings as a function of duration time (1–20 min) were investigated. It was observed that some granules started to appear in the electrolyte adjacent to anode after MAO duration for about 5 min and become more in amount with prolonging duration time. The obtained results show that the coatings consist of HA- and TiO 2-based two layers, in which the HA-based outerlayer contains small amounts of α-TCP (α-Ca 3(PO 4) 2) and CaCO 3 while the TiO 2-based innerlayer contains CaTiO 3. The granules are negatively charged amorphous HA, TCP and CaCO 3. It is suggested that formation of the granules in the electrolyte adjacent to anode, electro-migration of the granules onto the pre-formed TiO 2-based layer and sintering-caused phase transformation of the adhered granules are possible mechanism for HA-based layer formation.